Freezing point depression

i was just wondering why salt, when it dissociates into Na an Cl as it is dissolved into water, decreases the freezing point of water. Why does more energy have to be taken out of the system? does it have to do with entropy or what?

If you add some solute to an ice/water mixture in equilibrium in a thermostat, the ice will melt. If you do the same thing in a thermally insulated vessel, the temperature will decrease until a new equilibrium between ice and the solution is attained. In dilute solutions, the change in temperature is given by the van't Hoff equation:
dT = -x RT2/(Heat of fusion of ice),
where x is the mole fraction of the solute. The point here is that there will be depression of the freezing point for ANY solute, so the explanation cannot depend on the properties of a particular substance such as salt. What happens for large values of x will depend on the properties of the solvent and solute. For a pictorial "explanation" go to http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml

For a more advanced treatment, click on the link "How can freezing point depression be explained in terms of free energies":